1. Field of the Invention
The present invention relates to a video coding, and more particularly, to an apparatus for controlling a compression rate in a video coding, and a method therefor.
2. Description of the Related Art
In general, since video data having multi-dimensional characteristics has an information quantity larger than that of other data, the data quantity becomes huge when the multi-dimensional video data is processed digitally. Thus, in order to transmit and store the digitally processed video data, it is necessary to compress the data in consideration of the bandwidths of a communication channel required for transmission and the capacity of a storage medium required for storing information.
Standardization for moving picture video compression has been carried out by the Moving Picture Expert Group (MPEG) of JTC1/SC.29 which is a sub-committee of the International Organization for Standardization and the International Electrotechnical Commission (ISO-IEC). Particularly, standardization of MPEG-1 and MPEG-2 had been already completed and they are currently in use. Meanwhile, the International Telecommunications Union--Terminal Section (ITU-T) has made efforts to prepare video coding standards based on an Asynchronous Transfer Mode (ATM) protocol for video transmission in future broadband integrated services digital networks (B-ISDN) and has recently recommended several H.26x standards.
In a conventional coding scheme such as the MPEG-2, Differential Pulse Code Modulation (DPCM) or Discrete Cosine Transform (DCT) for reducing signal redundancy is used as a preliminary step for an effective coding. However, since a substantial compression of a signal actually occurs in the final quantization stage, the coding compression efficiency considerably depends upon the determination of a quantization step size rather than the DPCM or DCT.
To control a compression rate, the quantization step size in MPEG-2 is determined by activity calculation and buffer control. However, according to this method, picture quality may change depending on the position on a screen when a picture has both a complexity portion and an even portion in a frame. Also, since determination of the quantization step size by activities is processed based upon the luminance block having the minimum activity among four luminance blocks in a macroblock, the quantization step size suitable for human visual sensitivity is not determined due to incorrect discrimination of the even portion, edge portion and complex portion. Further, each of video objects constituting a picture is not taken into consideration individually, since the image signal is processed in the unit of frames.
In H.263, the same quantization step size is applied to whole frame, and the quantization step of the next frame is determined depending on the bit amounts of the previous frame. This method can be implemented in a simpler structure than MPEG-2. However, visual sensitivity is not still taken into consideration.
If the quantization step is determined without consideration of the visual sensitivity, the same quantization step size is applied to both more significant video objects such as human figure in a transmitted picture and less significant video objects such as background. Thus, signal compression effect may be outstanding, which greatly lowers picture quality. This is a severe problem in a low bit rate coding, particularly in the H.263 standard concerning video coding for low bit rate communications or MPEG-4 standards concerning very low bit rate video coding for mobile communications.
Therefore, to maximize the signal compression effect while minimizing deterioration in picture quality, it is necessary to determine the quantization step size depending on significance of several video objects constituting a frame in consideration of human visual characteristics and allocate appropriate bit amount to each video object.